PROJECT SUMMARY A major open - and very important - question in cognitive neuroscience is: how and where in the brain are decisions made? Recent electrophysiological evidence from a cerebral cortical area thought to play a key role in perceptual decision-making (LIP) suggests that the activity of neurons in area LIP do not reflect a gradual accumulation of evidence toward a decision. Furthermore, reversible inactivation of area LIP produces no change in decision-making performance, calling into question its causal role in visual perceptual decision- making. These unexpected results open the possibility that other sensorimotor areas play a critical role in the neuronal processing leading to perceptual decisions. Our goal is to expand on these recent findings and on our accomplishments made during previous award cycles to test the hypothesis that the basal ganglia (BG) and the superior colliculus (SC) play key roles in visual perceptual decision-making. We have two specific aims: 1) Determine the role of the superior colliculus (SC) in perceptual decision-making and, 2) determine the role of the inhibition from one output nucleus of the basal ganglia, the substantia nigra pars reticulata (nigra), to the SC in perceptual decision-making. We propose to use state of the art multiple neuron recording technologies to measure population activity in the SC and in the nigra while subjects perform visual perceptual decision tasks. We will employ analytical methods to model the behavioral data and make predictions about behavior using the population neuronal data. We will also employ cutting edge molecular genetic methods to manipulate neuronal activity to determine the causal role of these brain areas in perceptual decision-making.